1. Field of the Invention
The invention relates to a high-power radiator, in particular for ultraviolet light, having a discharge chamber filled with a filling gas which emits radiation under discharge conditions, the walls of said chamber being formed by an external and an internal dielectric and the outer surfaces of the external dielectric being provided with first electrodes, having second electrodes on the surface of the second dielectric remote from the discharge chamber, and having an alternating current source connected to the first and second electrodes for feeding the discharge.
In this connection, the invention proceeds from prior art such as is disclosed, for instance, by EP-A 254 111, U.S. patent application Ser. No. 07/485,544 dated Feb. 27, 1990 or, alternatively, by European Patent Application 90103082.5 dated Feb. 17, 1990.
2. Discussion of Background
The industrial use of photochemical processes is very dependent on the availability of suitable UV sources. The conventional UV radiators provide low to medium UV intensities at a few discreet wavelengths such as, for example, the mercury low-pressure lamps at 185 nm and in particular at 254 nm. Really high UV powers are obtained only from high-pressure lamps (Xe, Hg), but these then distribute their radiation over a larger wavelength range. The new excimer lasers have made a few new wavelengths available for fundamental photochemical experiments, but they are probably suitable at present for an industrial process only in exceptional cases for cost reasons.
The European patent application mentioned at the outset or, alternatively, the conference publication entitled "Noble UV and VUV excimer radiators" by U. Kogelschatz and B. Eliasson, distributed at the 10th Lecture Meeting of the Society of German Chemists, Specialist Group on Photochemistry in Wurzburg (BRD) on 18-20 Nov. 1987 describe a noble excimer radiator. This noble radiator type is based on the principle that excimer radiation can be generated even in dark electrical discharges, a type of discharge which is used on a commercial scale in the generation of ozone. In the current filaments of this discharge, which are present only for a short time (&lt;1 microsecond), noble gas atoms are excited by electron collision which react further to form excited molecular complexes (excimers). Said excimers live only a few 100 nanoseconds and give up their bonding energy in the form of UV radiation upon decomposing.
The high-power radiators mentioned are remarkable for high efficiency and economical construction, and make it possible to produce large radiators such as those used in UV polymerization and sterilization. In this connection, wide conveyor belts or conveyor cylinders often have to be irradiated by rod-type UV radiators. Typically, sheets, papers, cardboards, lengths of fabric, etc. coated with paints, lacquers or adhesives are irradiated by UV lamps approximately one meter long. Since the intensity of the lamps is normally distributed uniformly over the length, the peripheral zones of the substrate naturally receive a lower radiation dose. In order to obtain a dose sufficient for the process even at the periphery, the radiators have to remain substantially longer than the width of the substrate. This is usually out of the question in conveyor belt installations for design reasons. The other possibility is to increase the intensity of the lamps to such an extent that the dose is just sufficient at the periphery. Consequently, a substantial swamping of the central zones with light is acceded to, with a corresponding energy consumption.